Combining Quarks into Hadrons

The "particles" setter summarizes the component particles of the Standard Model: quarks, leptons, gauge bosons, and the putative Higgs particle.

The fact that isolated quarks have never been observed is referred to as "quark confinement". Baryons and mesons, known collectively as hadrons, exist as combinations of quarks. Baryons, including the proton and neutron, consist of quark triplets. Mesons combine a quark with an antiquark. There also exist antibaryons, made of antiquark triplets. All mesons and baryons, with the exception of the proton and the neutron (when part of a nucleus) are unstable and decay into more stable particles with lifetimes ranging in order from to seconds. The six flavors of quarks are named up (u), down (d), strange (s), charm (c), bottom (b), and top (t). Quarks have fractional electric charges, with for u, c, and t, for d, s, and b (in multiples of the electron charge e). The corresponding antiquarks have charges with the opposite sign.

All quarks (and antiquarks) have spin 1/2, like the electron. They therefore behave as fermions. As a consequence, all baryons have odd-half-integer spins (1/2, 3/2, etc.) and are fermions, while mesons have integer spins (0, 1, etc.) and are bosons.

In this Demonstration, you can create combinations of u, d, and s quarks and antiquarks to synthesize some of the lower-mass baryons and mesons. Since, in the domain of elementary particles, mass and energy are essentially equivalent (), quark and hadron masses are conventionally expressed in units of .

Each quark and antiquark flavor comes in three colors, which serve as the "charges" for the strong interaction. Baryons are "color neutral" combinations of red, blue and green quarks. Mesons achieve color neutrality by combining a color with its "anticolor". If not for color, combinations such as uuu with total spin 3/2 could not exist without violating the Pauli exclusion principle. Quark color is not otherwise considered in this Demonstration.

THINGS TO TRY

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Snapshot 1: The Standard Model "cast of characters".

Snapshot 2: The proton has spin 1/2, quark composition uud. Its charge is therefore given by , the negative of the electron charge. The higher energy hyperon contains the same quarks, but with total spin 3/2.

Snapshot 3: The spin 1/2 combination udd gives the neutron, with . Although this is a neutral particle overall, it has internal positive and negative charges, which account for its magnetic moment. The hyperon has the same quark composition, but total spin 3/2.

Snapshot 4: One of the multiplet of three pions, , , and , made of u and d quarks and antiquarks.